Redwoods are among the most studied trees on the planet, yet their mysteries continue to surprise and delight scientists and nature lovers alike.
Scientists from the University of California, Davis have found that redwood trees have two types of leaves and that these leaves have completely different functions, according to a study published in American Journal of Botany. Together, these functionally distinct leaves allow the world’s tallest trees to thrive in both wet and dry parts of their California range, without sacrificing water or food.
Division of labor
The peripheral leaf spends its working hours making food for the tree, converting sunlight into sugar through photosynthesis. Its colleague, the axial leaf, does almost nothing to help with photosynthesis. Instead, his specialty is absorbing water. In fact, the study found that a large redwood tree can absorb up to 14 gallons of water in just the first hour its leaves are wet.
How does this compare to other trees? Scientists don’t know. This is the first study to assess water uptake throughout the crown of a large mature tree. Because tall redwoods have over 100 million leaves, this absorption record can be hard to beat.
In humid forests, photosynthesis can be inhibited by films of water covering the stomata of leaves when they are wet. For redwoods, the different types of leaves allow the trees to get wet while still being able to photosynthesise. The peripheral leaves have a waxy coating which slows water uptake but may help them to continue photosynthesis throughout the rainy season.
“I’d be surprised if there weren’t a lot of conifers doing this,” said lead author Alana Chin, who holds a Ph.D. an ecology student in the Department of Plant Sciences at UC Davis at the time of the study. “Having leaves that aren’t meant for photosynthesis is in itself surprising. If you’re a tree, you don’t want to have a leaf that doesn’t photosynthesize unless there’s a very good reason why. that.”
The study also found that leaves can shift their “office space” along the tree depending on whether the environment is wet or dry.
On the humid and rainy north coast, the water-absorbing leaf type is found on the lower branches of the tree, leaving the sunnier upper levels to the photosynthesizing leaf type. This dynamic is reversed for redwoods in their southern range: water-collectors live among the upper levels of the tree to take more advantage of fog and rain, which occur less often in the drier environment. .
To arrive at their conclusions, the authors collected shoot clusters from six redwood trees in five forest areas stretching from wet Del Norte County to the dry Santa Cruz Mountains and exposed them to an experimental fog. They estimated the water uptake potential of seven additional trees – including the tallest living tree – and took samples at different heights.
They then compared the anatomy and measured the photosynthesis of peripheral and axial leaves to understand their function. They also developed a physics-based causal model that allowed them to determine leaf traits that regulate uptake rates.
Among all the discoveries, Chin is very happy to have found a simple and effective way to indicate the ability of redwoods to access fog. Researchers can monitor how and if redwood trees are adapting to climatic conditions and a drier future world by simply looking at the visible waxes covering both types of leaves – something that could be captured on a cell phone camera and shared by other scientists or even members of the public.
Redwoods are renowned for their resilience in the face of many natural threats and inspire many superlatives: they are among the tallest, tallest, and oldest trees on the planet. They have a tannin-rich heartwood, fire-resistant bark, and pest-resistant leaves. This new discovery is another example of their ability to respond to environmental conditions, such as drought and water stress.
“What’s cool here is their ability to thrive in all of these circumstances and adapt to these different environments,” said Chin, who grew up near the redwoods of Mendocino County. “That things like this could happen right under our noses in one of the best-studied species – none of us thought that would be history.”
Study co-authors include Paula Guzman-Delgado, Jessica Orozco, Zane Moore, and lead author Maciej Zwieniecki of UC Davis’ Department of Plant Sciences, and Stephen Sillett, Lucy Kerhoulas, and Marty Reed of Cal Poly. Humboldt and Russell Kramer of Dipper and Spruce LLC in Washington.
The study was funded by the National Science Foundation and a Katherine Esau Fellowship from UC Davis.